Some chimps are more outgoing than others. Some like trying out new foods and games while their friends stick to the tried and tested. In short, chimps have different personalities, just like people do. What’s more, psychologists investigating chimp personality have found that their traits tend to coalescence into five main factors, again much like human personality. Three of these factors are actually named the same as their human equivalents: Extraversion, Openness and Agreeableness. The other two are Dominance (a bit like the opposite of the human trait of Neuroticism) and Reactivity/Undependability (opposite to the human trait Conscientiousness).

Now a team of psychologists and primatologists has scanned the brains of 107 chimpanzees to try to find the neural correlates of personality differences in our evolutionary cousin. The neurobiological basis of human personality is a thriving area of research, but this study published in NeuroImage is the first to look for the brain basis of chimp personality.

The chimp participants were residents at The University of Texas MD Anderson Cancer Center, housed in groups of 5 to 14. There were 50 males with an average age of 22 and 57 females with an average age of 20. Robert Latzman and his colleagues relied on colony staff to rate the chimps’ personalities using a 41-item personality questionnaire which tapped the chimp equivalent of the five main personality traits. The chimps then had their brains scanned by MRI, for which they were sedated.

Given the evidence in humans that that has linked many aspects of personality to features of the frontal cortex, the researchers decided to focus their investigation in that part of the chimp brain. After controlling for age and sex (older chimps were less reactive and less extravert; males tended to be more extravert and dominant), they found that the more grey matter volume a chimp had in his or her frontal cortex, the more dominant, open and extravert the chimp tended to be. The researchers said this potentially reflects the broad role of the frontal cortex in “the control of emotions in the service of goal-directed behaviour”.

Zooming in on a particular sub-structure in the frontal cortex, the anterior cingulate cortex (ACC; an area associated in humans with motivation and expectations, among other things), higher Extraversion and Openness were associated with more grey matter in this structure. The researchers also looked at asymmetries in grey matter volume between the chimps’ two brain hemispheres. Here they found that more grey matter volume in the right hemisphere was associated with higher Extraversion and Dominance, which contradicts human research which has linked approach behaviours with the left-hemisphere. However, when the researchers looked at asymmetries in specific structures, including the ACC and the medial prefrontal cortex, they found that more grey matter in the right hemisphere was associated with more reactivity, while a left-hemisphere bias was associated with more dominance, which is more consistent with human evidence.

The study makes a good to start at exploring the neurobiology of chimp personality but it does have some problems, including the cross-sectional design (were the brain differences a cause or consequence of personality differences?), the exclusive focus on the frontal cortex, and the way the researchers translated each chimp’s brain structure onto a common template, thus losing some of the individuality between chimps. Nonetheless, Latzman and his colleagues said their findings added further evidence to the idea that human personality has an evolutionary and biological basis, and confirmed “the importance of neuroscientific approaches to the study of basic dispositions (i.e. personality) … suggest[ing] that many of these associations are comparable in chimpanzees.”_________________________________